LNG Process Risk Safety Modeling Thesis

Excerpt from Thesis :

The main hazards related to LNG include:

Rupture due to Corrosion

Rupture while excavation

Rupture while excavation

Rupture during an earthquake

Rupture due to mechanical failure

Rupture at compressor

Rupture at inspection stations

Uncontrolled detonation of explosives

Blow-out of gas at head and subsequent fire

Gas leak from infrastructure

Fire involving combustible

Construction damage

LPG or Diesel

Diesel pump fire involving equipment brittle fracture valve Leaks

Welding failure welding casting failure

Mechanical overstressing of equipment Vibration

pump Corrosion

joint Erosion

Failure due to external loading or impact

Internal Explosion

Underground pipe rupture of transmission pipeline

Pipe rupture at main line valve sites.

Rupture of adjacent gas pipeline

Uncontrolled detonation of explosives

Gas leak from pipeline infrastructure

Drop of pipe from pipe lifts

Accommodation fire involving combustible construction LPG or Diesel

Diesel fire involving mobile fuel tanker

Uncontrolled release of LNG

Uncontrolled release of refrigerant gas

Uncontrolled release of by- product toxic gases (e.g. H2S, CO, CO2)

Plant fire involving pressure vessel of hydrocarbons

Uncontrolled release of product on production

Fire in process plant (e.g. Cable, lubrication oil, transformer etc.)

Gas explosion during maintenance or decommissioning

Fire from vapor cloud ignition during well operation

Fire from condensate ignition during well operation

Fire during well drilling

Liquid diesel release during well drilling

Fire or explosion of gaseous hydrocarbons at Production Facility or Hides Gas Conditioning Plant during operation

Fire, involving hydrocarbon liquids

Fire at Production Facility or Hides Gas-Conditioning Plant during construction

Explosion or fire along an onshore gas pipeline

Liquid hydrocarbons spill along an onshore liquids pipeline

Fuel spill during construction of onshore pipelines

Loss of liquid containment from the inner tank followed by a pool fire in the bund

Loss of vapor from the outer tank due to overpressure condition with Ignition

Condensate or LNG spill or vapor release during ship loading

Vessel grounds during inbound or outbound transit

Collision of LNG carrier, condensate tanker or tugboat with fishing boat long loss

LNG spill

An analysis of these hazards reveals that they have different severity indices in relation to the extent of damage they can cause to the facility, community and the business. Their rates of probability also vary. Their failure effect and hazard rates also vary. The failure effects of the various LNG hazards range from 5% to 90% which are considered critical and severe respectively in terms of severity class. This shows that a lot of care must be take to curtail this wide range of hazards resulting due to LNG incidents.

Risk Analysis

This is a systematic utilization of the available information used for the identification of hazards so as to estimate the level of risk to individuals, population, environment as well as property

Risk Assessment

The overall process that is involved in the risk analysis as well as risk evaluation and usually compares the risk analysis estimates

The Liquid Natural Gas Process Chain

It was until 1964 that the Liquid Natural gas followed a process of production, import, distribution and export that followed a due sequence as illustrated in the figure below.

The Processing of LNG form extraction to consumption (Source BV -2009)

The first step in the processing chain of a natural gas is extraction. Most countries with the large natural gas reserves export this product to other countries with no reserves. The total number of these countries is 15 but the total number of the LNG plants was 22 by the beginning of the year 2008. These countries include: Indonesia, Algeria, Egypt, Russia, Qatar, Yemen, Malaysia, UAE, Nigeria, Australia, Trinidad, Brunei and Norway. Although USA also produces the natural gas, it is mainly for domestic market as their reserve is not adequate to allow exportation. In most cases the gas supply may not be enough to meet intra-country needs hence the countries import the deficit from the countries with surplus.

Once a team of Geologists and geophysicists locate a field with potential to produce the gas, a special team is sent to drill the point the prospective field to establish the viability of the quantity of the gas and if verified, the next procedure is extraction as well as processing. It is important to note that before the commercial market of the gas was established, the gas which was associated with oil, was wasted in a flare but now its value has been established and being used as LNG. There is a procedure that the natural gas must pass through in order to be fit for sale to remove impurities that are usually associated with Natural gas, which is mainly methane. Such impurities include: ethane, propane, hydrogen sulfide (H2S), Carbon dioxide (CO2), Butane, Pentane, Helium and Nitrogen as well as water and oil. These impurities must be eliminated before liquefaction to become LNG.

The Liquefaction Plant

The second stage in the process is cleaning at the liquefaction plant where a series of processing steps ensure the removal of the impure and extraneous compounds from the raw material just before liquefaction.

Purification of the product

The main reason why this purification process is necessary is that before the LNG is loaded onto tankers, trucks or ships for transportation, the composition and combustion properties must be consistently provided. This is achieved through cooling and condensation of the gas. The consistency n the content of the LNG is critical so as to obtain pipeline-quality gas which usually contains between 86% - 99% methane. It is normally associated with long-chain hydrocarbons and other impurities that fail to be removed during the processing. The figure 2 below summarizes the stripping process of removal of the compounds from the natural gas as it leaves the ground before the start of liquefaction process.

The flow process for natural purification before liquefaction (source BV -- 2009)

Carbon dioxide and water are usually removed prior to liquefaction since they can cause the malfunctioning of the liquefaction equipment due to freezing properties. Hydrocarbons with longer carbon chain like ethane, propane butane and pentane are also removed and sold as fuel to petrochemical industries.


After the removal of most impurities and long-term hydrocarbons, the gas that results is mainly methane that is ready to undergo the process of liquefaction. A refrigeration technology, which is able to cool of the gas to temperatures as low as -162oC (-259oF) is used. When it liquefies, the LNG becomes a non-corrosive liquid which is as colorless as water 50% less than the weight of water. That is to say, it is half less dense than water. The LNG is more portable than the natural gas to transport since one volume of LNG is equivalent to 600 volumes of natural gas at standard temperature and pressure. This is what makes economically lucrative to transport by truck or ship.

The authors Aldwinkle and Slater (1983) had a discussion of risk and reliability analysis of the appropriate methods to be used for certain type of offshore LNG terminals, liquefaction as well as the storage ships that are secured via a single point mooring attached to an underground pipeline (Woodward and Pitblado,2010).The use of other conventional risk assessment as well as reliability methods is presented with a caveat regarding the fact that it is very difficult to estimate the failure frequencies. The tree analysis can however be used in this case provided there is relevant data. This is used in conjunction with failure mode and effect analysis (FMEA) so as to relate the consequences of the various postulated failures.

Below are the major systems that have been identified for the fault tree analysis that leads to major events of LNG gas leakage and spillage;

Liquefaction process plant failures

Single - point mooring failures

Containment system failures

LNG transfer arm failures and LNG piping system failures

They are used in the analysis of the cost/benefit ratios for the mitigation measures that are proposed. For the single - point mooring failures scenario, the mitigation usually consists of the measures that are aimed at protecting the collision of shuttle ships involved in the loading the LNG for the subsequent delivery to various markets. The options to be evaluated in this case are;

Collision barrier

Sacrificial structure.

This involves the calculation of the structure's energy - absorbing capability. This technique can be used to effectively illustrate how risk analysis can be employed in order to design structures.

Other notable contributions have been advanced for the concept of FLEX LNG and the concept of Hoegh LNG as pointed out by Pastoor et al., (2009); Festen and Leo,(2009); Iversen and Hellekleiv,(2009 ).

Transport of LNG

There are three modes of transporting the LNG which are: sea, rail and like Japan use rail. In the sea, it is transported by using specialized LNG carriers. The first transportation of LNG was done in 1959 in the Lake Charles in Louisiana and was destined to Canvey Island in the United Kingdom. The name of the voyage was MV methane. The initial stages of using the sea…

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